In the past, various fluids have been used as heat exchange media for heating other fluids such as air, water and natural gas. At present, probably the type of heat exchange medium most commonly used in gas transmission lines, for example, is a glycol such as ethylene glycol, diethylene glycol, triethylene glycol, or a mixture of them. These are favored because they not only have good heat exchange properties but also have low freezing points and/or can depress the freezing points of mixtures with water. A low freezing point is desirable during periods when the heat exchange fluid is idle, or it leaks, is outdoors in storage, or it is removed from the system for any reason. At present, ethylene glycol (EG) is probably the heat exchange fluid most commonly used in heaters for the natural gas in gas transmission lines. Such heaters are most commonly called line heaters.
Prior to the present invention, line heaters have been placed between compressing stations on gas transmission lines. Heating the gas in remote locations on a gas transmission line and/or between compression stations has more than one beneficial effect. First, the heater assists in moving the gas through successive pumping or compression stations by adding energy to it, reducing the density, and utilizing the tendency to expand to assist in the forwarding of the gas from station to station. Second, heating minimizes the tendency of moisture to condense from the gas on cooling. Liquid moisture in the pipeline is undesirable not only because it can facilitate corrosion but also because it can freeze and sometimes cause blockages or resistance to flow.
Exemplary line heaters are known in the art and are described in “Gas Conditioning and Processing—The Equipment Modules”, pages 82-86, originally attached as Exhibit A as part of Provisional Application Ser. No. 60/184,146. The line heaters employ coils, tubes, or other means for segregating the heat exchange fluid from the natural gas, and may be either direct fired or indirect fired. We prefer to use indirect fired line heaters, in which a source of heat such as a firetube is held within a vessel also including a coil and/or tubes for circulating the natural gas to be heated. The heat transfer fluid occupies a substantial volume in the vessel. After it is heated by the firetube in the heat exchange fluid heating zone, the heat exchange fluid is transported to a gas heating zone, where the coil and tubes are heated containing the circulating gas. The tubes can be substantially horizontal, radial, or any other configuration for efficient heat transfer, and may be made from various materials generally chosen for their heat exchange properties, as is known in the art. FIG. 1, based on Exhibits B and C of Provisional Application Ser. No. 60/184,146, depicts a common design for an indirect heater as known in the art.
Alternatively, the heat exchange fluid may be contained within coils, tubes, and/or other containers which are usually heated by flame in a heating zone, and circulated within the coils, tubes or the like to a natural gas heating zone, where the heated fluid gives up its thermal energy to the natural gas. It is not essential that the original (heat transfer fluid) heating zone be near the gas heating zone, and in fact the two heating zones can be in separate pieces of equipment while the heat transfer fluid is moved from one zone to the other. The heat exchange fluid is more or less continuously circulated while the heating process is conducted. In other fluid heaters, pumps may be used to move the heat energy from its source through the heat exchange fluid to the fluid to be heated.
It is desirable to find an environmentally more acceptable material for the heat exchange fluid in line heaters and for other fluid heater applications because, when the heat exchange fluid is leaked even in relatively small quantities onto the earth during an accident or otherwise, the mishap must be reported to the Environmental Protection Agency and/or to other regulatory agencies, and resources must be diverted to correcting any problems caused by the leak.
The reader may be interested in reviewing U.S. Pat. No. 5,104,562 to Kardos et al, which describes aqueous combinations of potassium formate and potassium acetate for use as cooling compositions.